The World Cup is held every 4 years. Until 1994, this is how it worked:
45 mins 1st half
15 mins break
45 mins second half
At the end of this 90 minute period, if the game was still tied, whether at 0-0 or 5-5, it was necessary to determine a winner by some other means.
So, there was Extra Time:
15 mins 1st Half of Extra Time.
15 mins 2nd Half of Extra Time.
If the score was still tied, there was a Penalty Shoot-Out.
There was a possibility that the 0-0 match was now tied at 1-1 at the end of Extra Time, therefore requiring a Penalty Shoot-out.
Now, a Penalty shoot-out is the Russian Roulette of Football, and hence not particularly favoured by players and fans alike. So FIFA was looking for a way to determine the winner before reaching this stage.
So they introduced what is called a Golden Goal (after testing the rule at various levels of international football)
This rule is simple. In the Extra Time first team to score a goal wins. This goal is known as the Golden Goal. Match ends, whether the goal is scored in the first minute of the 30mins extra time or the last minute. This prevents a penalty shoot-out and also the scenario where a 0-0 score after normal time ends as 1-1 at the end of extra time, since at 1-0 the match would be over.
As mentioned by ruadh, this was introduced as a means of encouraging attacking play, but has caused many teams to play defensively instead, and therefore FIFA may change the rule in the future.
The reason that a Golden Goal is so rare in a World Cup is because:
It was introduced in a World Cup for the first time in 1998.
Out of 64 total games played in the 1998 World Cup, the first 48 matches are league matches and hence a tie is a valid result and no extra time is played at all.
Of the remaining 16 matches at the 1998 World Cup, 12 were decided at the end of normal playing time.
That leaves just 4 matches with the possibility of even using the Golden Goal rule. Of these, 3 of them ended up with no goal scored in the 30 mins of extra time, and thus were decided by a penalty shoot-out.
That left 1 match (France vs Paraguay), which was decided by a Golden Goal. The first Golden Goal. Scored in the 8th minute of the second half of Extra Time by Laurent Blanc of France.
In the current 2002 World Cup, out of 64 matches the 48 matches of the group stage (league matches) could end in a tie being a valid result, and hence did not require the golden goal rule.
Of the remaining 16 matches, only 6 have been played so far (as of writing of this answer). Of these 6, only 2 matches needed Extra Time. The Senegal vs Sweden game, which ended in a Golden Goal by Senegal, the second ever. And the Spain vs Ireland game, which was tied 1-1 at the end of normal playing time, and remained tied at 1-1 at the end of Extra Time, and hence went to Penalty Shoot-Outs with Spain winning.
The reason why a Golden Goal is not called Sudden Death is because:
-
Extra Time is still called Extra Time.
-
At the end of Extra Time, if the match is still tied, a Penalty Shoot-out takes place. 5 Penalties per team, taken alternately. If, at the end of the designated 5 penalty kicks the score is still tied, the game enters Sudden Death:
One Penalty kick each team, taken alternately. Repeated until one team misses AND the other scores (as Crusoe states).